Crabgrass is an invasive summer annual that can quickly dominate a healthy lawn. Its aggressive nature and rapid spread make it a frustrating challenge for turf management. This weed produces a vast number of seeds and thrives in conditions that stress desirable turfgrasses. Understanding the life cycle and physical traits of this plant is the first step toward effective management.
Physical Characteristics and Common Species
Crabgrass belongs to the genus Digitaria, with large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum) being the most prevalent species in North America. Both species have a coarse texture, light green color, and a characteristic sprawling growth pattern. The plants grow low to the ground from a central point, sending out multiple stems or tillers that resemble crab legs. Large crabgrass is distinguished by sparse hairs on its leaves and broader blades, while smooth crabgrass is smaller and generally lacks hairs. As the plants mature, they produce finger-like seed heads clustered at the top of the stems.
Understanding the Annual Growth Cycle
Crabgrass is a warm-season annual, meaning its entire life span occurs within a single growing season. The cycle begins in spring when soil temperatures consistently reach 55 to 60 degrees Fahrenheit at a one-inch depth for several consecutive days. This germination window is a reliable indicator of when the weed will emerge. Once seedlings emerge, the plant enters a period of rapid growth throughout the summer months, outcompeting cool-season turf when temperatures rise. The most problematic stage occurs in late summer and early fall, when each plant can produce up to 150,000 seeds that remain dormant over winter until the cycle restarts the following spring.
Effective Control and Prevention Methods
Effective crabgrass management relies on a combination of cultural practices, pre-emergent treatment, and, if necessary, post-emergent control. Cultural control focuses on strengthening the lawn to make it a hostile environment for weed germination.
Cultural Practices
Mowing the lawn at a taller height, typically between three and four inches, helps shade the soil surface. This keeps the soil cooler and less favorable for crabgrass seeds. Deep, infrequent watering encourages desirable turfgrass to develop a deep root system, helping it withstand summer heat. Maintaining a dense turf through proper fertilization, particularly in the fall, is the most effective long-term preventative measure, as thick grass crowds out space for germination.
Pre-Emergent Herbicides
The most successful chemical strategy is using pre-emergent herbicides, which must be applied before the crabgrass seeds germinate in the spring. These products form a chemical barrier at the soil surface that kills the seedling as it attempts to emerge. Accurate timing is paramount for success. The application must be watered in lightly to activate this chemical barrier.
Post-Emergent Herbicides
If the timing for pre-emergent control is missed, post-emergent herbicides are necessary to kill established plants. These products are most effective when the crabgrass is young, ideally in the two- to five-leaf stage, before it has produced many tillers. Products containing the active ingredient Quinclorac are often used for selective control. They target the crabgrass without harming the surrounding desirable turfgrass.